Why do you boys come back after the girl you done wrong moved on?

he started talking to this 18 year old girl as a rebound. me and the guy both are in our 20's. the 18 year old moved in with him she's not in school but he's mostly getting money and sex and house service from her. I repeatedly tried to fix things between us he pushed me away and said he had a girlfriend and that I'm stupid and I'm a sad *** female and a sad *** excuse for a woman...he only give her the phone to answer when he mad at me...he thought I was not gonna move on and that I was gonna be stuck on him forever...i finally moved on to another guy and he tweeted on twitter his plan is failing...why do guys come back after they done you wrong? I use to always text and call him everyday trying to fix things even become friends again...nothing he pushed me away...when you move a girl in that's it for me..they been dating for 6 months her expiration date is June and she moved in with him not even a month after she met his parents...the girl he with not even in college it took her 3 months to work...shes a waitress she can't drive the only thing she can do is lay around him depending on him...she moved right out of her mom house into his...i been with this guy since 2005